The goal
Fill every empty cell so each row, each column and each 3×3 box contains the digits 1 through 9 exactly once. The printed numbers are givens and cannot be changed.
1. Scan rows, columns and boxes
Choose an area that is nearly complete. List the missing digits, then check the crossing row and column for each empty cell. If only one missing digit can legally occupy a cell, place it.
2. Add candidate notes
When a cell has several possibilities, write those digits as small candidate notes. A confirmed placement removes that digit from every peer in the same row, column and box.
3. Look for singles
A naked single is a cell with only one candidate. A hidden single appears when a digit has only one possible position within a row, column or box—even if that cell currently has several notes.
Try it: spot the naked single
Walk through a real deduction, one step at a time.
4. Repeat after every placement
Sudoku progresses in cycles. Every confirmed digit changes its peers, so rescan the affected row, column and box before searching elsewhere.
Start your first puzzle
For concise constraints, read the Sudoku rules. For the next stage, explore the Sudoku techniques guide.